How to Test a Car Alternator

Daniel McDonald
Daniel McDonald8 min read
How to Test a Car Alternator

One of the most frequently asked questions we get from new car owners is how they can test a car alternator. Many signs can indicate that your car's alternator is not running at an optimum range.

The trickiest thing about it is that it can be hard to tell whether you have a bad alternator or you're facing some other problem, but before we get into the details it is important to know the basics about an alternator.

What is a Car Alternator?

An alternator's job is to power different parts of the car; such as the radio, headlights, and other electrical parts. The car alternator helps keep the vehicle running and ensures a swift journey.

Just as a car needs a battery voltage to start, a car alternator is necessary to ensure the engine runs swiftly. The alternator also provides the electrical components with DC power, which keeps them running. The same current charges the battery while you drive.

In most cases, alternators can last a long time but eventually, they may start deteriorating. Factors such as wear and tear, heat damage, and faulty wires can contribute to deterioration. Ultimately, your car will need a new alternator if the current one isn't producing enough power.

Signs of a Bad Alternator

An alternator is a core component of your car. Without a compact alternator, your car won't start, and even if it does, it will only stay on for a few moments. Typically, car owners confuse alternator problems with a dead battery and low battery voltage.

To help you out, we have identified a few notable signs of a dysfunctional alternator.

  • Dim Lights

One of the first signs that could indicate a dysfunctional alternator is that your lights are getting inconsistent voltage. If you notice flickering lights, it's a sign that your car needs a new alternator.

  • Dead battery

You may think that the battery is dead because it has reached its limit or you left the headlights on, but it could just be a malfunctioning alternator.

If the alternator isn't running at an efficient pace, it may not charge the car battery as it should. One of the ways you can identify a dead battery is by jumpstarting the car.

  • Frequent Stalling

If you are having trouble starting the vehicle, then it may indicate that your alternator needs replacing. Often, frequent stalling means that there is not enough voltage in the battery.

  • Growling Noises

Your car's engine compartment may produce different noises while you drive. Some of these noises are harmless, but others can pose significant problems. If your alternator isn't working properly, then you may hear growling or whining noise from under the hood.

  • The smell of Burning Tires or Wires

If you can sense the foul smell of burning wires or rubber, it's a sign that your alternator is beginning to wear out. The alteration drive belt is usually near the hot running engine, which exposes it to constant friction and tension.

Another sign of potential damage may include damaged and frayed wires, which explains the smell of burning wires. This usually occurs when the alternator's wires exceed their electrical capacity, causing them to heat up. You will detect an unpleasant burning smell when this occurs.

  • Battery Warning Lights

Usually, a battery warning light is associated with a battery and voltage issue. However, it can be more than that. These warning lights may indicate a major problem with the electrical system, including the alternator.

Since the alternator works at a specific voltage, a complication may result in an overall drop in the voltage capacity. This causes the battery warning lights to start flashing. The voltage fluctuation may depend on the car's voltage requirement.

How to Test your alternator?

In this guide, you will learn everything you need to know about testing your alternator.

Check Your Battery

The first thing that you will need to do is test your battery with the help of a v0ltmeter or a multimeter. The multimeter measures the voltage in the battery and gives you an accurate reading.

Make sure that the positive battery terminal and negative terminal are both clean. Connect the voltmeter with the right terminals and measure voltage. Note down the voltage range on the voltmeter.

If the range is between 12-13, then the alternator is powering the battery correctly.

Start the vehicle

The next step is to start the vehicle and note down the readings. If the number is anywhere between 12 volts to 17.0 volts, then the alternator is charging at the ideal range. Then, you will need to turn on the electrical system, including the headlights, radio, and fog lights.

When you turn on all components, make sure to check that the voltage doesn't drop too low. You will notice a gradual decrease but if it does not drop to 12.6 volts, then the alternator is working properly and is charging the battery.

Now, turn off the vehicle and notice the reading once again. This test allows you to determine and diagnose the issue with your alternator. You can also repeat the process for better accuracy.

If the test results are not as planned, prepare to look out for a few things.

For starters, make sure that the leads are properly intact with both battery terminals. This will ensure a good connection between the battery. A poor connection will negatively impact the alternator and lead to poor functionality. Therefore, it is important to make sure of the connections.

Another spot where you can look is the back of the alternator. In this area, you need to make sure that all wires are secured properly. You should also look out for corrosion or damage from wear and tear in this area.

The alternator also has a voltage regulator. Check the wires and make sure they are all connected and the external voltage regulator is running the right way.

The last thing you need to ensure is that the alternator is rotating. Make sure that the alternator belt is rigid and tight.

Battery Voltage Drop Tests for Positive Terminal and Negative Battery Terminals

The last test is the voltage test for the two sides of your battery. First, we will go with the voltage measurement for the negative side of a car battery.

Testing Negative Terminal

To perform this test, you need to turn on the car and the electrical accessories such as headlights and a blower fan. Connect the voltmeter's black lead to the negative side of the battery. And then, touch the red wire with the alternator case.

Once they are both touching, they will give you a reading. To get an accurate reading, you will need to ensure an effective connection. Once you start seeing the results on your voltmeter, it is time to increase the rpm to 1500.

If the reading is below 0.1 volts, your alternator is in good shape. However, if it is anywhere above that, you might be facing a complication.

Let's say that you get a reading of 0.1 volts or above. In that case, you can adopt a few measures. The first thing that you can do is take the red wire and attach it to the car bracket found in it.

If you see a big drop in voltage, it might indicate that there is a problem with the alternator. But if that's not the case, you can try the engine block. You have to take care of the engine running along with the fan.

There are a few other methods that you can adopt as well. Connect the red lead to any part of the engine to get a reading. Then again, if you see a significant decrease in the results, there could be a problem with your alternator.

The main idea is to get your alternator tested with all the ground connections. It will help you identify where the connection is bad. Make sure to set the RPM to 1500 when conducting these tests. You can perform re-tests with a voltmeter to get accurate results.

Once you identify where the connection is poor, you can make quick repairs.

Testing the Positive battery terminal

Similar to the previous method, you will need to get adequate accessories and run the car at 1500 rpm. Make sure to attach the red wire to the terminal.

Check the results in the voltmeter and make sure it is not above the 0.3-volt mark. Anything below the 0.3 volts mark is a good sign.

The next area where you may need to check for a voltage drop is the lead that's attached to the alternator. Make sure to keep testing until you find the problem area. When you find the connection where the problem lies, sand it off and clean it.

The alternator issue is common in older cars as opposed to new ones. By adopting a simple test and additional voltmeter testing, you can make sure your car engine runs smoothly

Keep checking the voltmeter for fluctuations in the voltage. While testing, keep your car engine running. Sufficient voltage is necessary for a healthy car alternator. You can easily find a cheap voltmeter in the market.

Alternative Method to Test the Alternator

There are some different methods that you can use to determine whether your alternator is working:

  • Check the Alternator Gauge

With the help of a volt gauge, you'll be able to determine the overall output of the alternator. To start out, you'll need to run the engine at 2000 rpm and turn on the blower fan with all the electrical components in the car.

If the output value of the voltmeter is higher while the engine is running, you can rest assured that the voltmeter is charging the battery.

  • Listen to the Alternator

Another way to test the alternator is by listening to its noise. You might hear squealing sounds from the car engine. And as you turn on the car's electrical system, the noise level increases.

You can even hear these noises while your car is running. Less noise is an indication that your vehicle is in good condition.

  • Turn on the radio when the engine is running

When you turn on the radio as your engine is running, you may hear a fuzzy noise. In this case, the alternator is causing the problem.

If nothing works out and you are unable to solve the alternator problem, you'll need to call a professional mechanic to deal with the issue. A professional mechanic can help you determine whether it's a battery fault or an issue with the alternator. Usually, they perform additional tests with the latest tools.

Furthermore, they also diagnose issues with the vehicle's charging system and battery.

Frequently Asked Questions

Whether it is a negative battery cable or a car's charging system, users have plenty of questions about alternators. Let's take a look at some notable concerns:

What does a Voltage Regulator Do?

The voltage regulator ensures that the car gets the required voltage at all times. It does so by rapidly switching the alternator in the charging circuit.

What Happens when you Disconnect the Negative Battery Cable?

When you remove the cable, there will be no current flowing in the cables. There will be no power in the overall system and the voltage remains null.

How do Computer-controlled charging systems work?

With the help of a computer, charging systems can provide sufficient speed, draw power, and charge by keeping in mind the temperature and total battery charged.

What Problems do a Loose Serpentine belt cause?

A loose serpentine belt can cause significant complications including engine problems. Therefore, it is important to replace the serpentine belt when it starts to loosen.

Final Thoughts

To identify any complications with your alternator, you need to ensure optimal battery voltage stays optimum. Moreover, you need to ensure other electrical properties in your car are running swiftly.

We hope this guide helped you know how to test your alternator and check it for problems. Keep in mind that you should contact a professional mechanic if you're unable to diagnose and solve the issue yourself.